System and method for controlling continuous mixer with melt pump

ABSTRACT

An improved system and method for effectively controlling the operation of a continuous mixer/melter having an adjustable exit opening and connected to a melt pump is provided. The melt pump, which is preferably a gear pump, is operatively connected to an extruder/pelletizer. Operation of the gear pump is controlled in proportional relation to the energy requirements of the mixer/melter motor, the energy applied to adjust and maintain the exit opening of the mixer/melter, and the suction pressure measured at the interface between the mixer/melter and the gear pump. A microprocessor unit which receives and processes the input signals, calculates the energy requirements of the gear pump motor and compares these requirements with the actual torque, makes the necessary adjustments to the gear pump.

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to a process for effectively controllingoperation of a continuous mixer having an adjustable exit opening whichis connected to a melt pump.

2. Description of the Prior Art

Numerous polymer resins, e.g., polyethylene and polypropylene, areproduced as very fine particles and are subsequently converted topellets to facilitate shipping and handling. At this stage of theprocessing, the resin can additionally be compounded with additives,such as stabilizers, antioxidants, fillers, colorants, and the like, ifdesired. In general terms, this in-line processing is accomplished bymixing and melting the resin after it has been purged of any unreactedmonomer(s) or other volatile materials and forcing the melt through asuitable die plate into a pelletizer. The pelletizer consists of a meansfor chopping and cooling the extrudate.

More specifically, the resin is continuously fed into a combinationmixer/melter having interspaced entrance and exit openings and a meansfor moving the material therethrough. Combination mixer/melters,sometimes simply referred to as mixers, are known and widely utilizedfor the continuous processing and compounding of thermoplasticmaterials. Mixer/melters have a restraining means for controlling theflow of exiting resin. This restraining means which in addition tocontrolling throughput, creates a back pressure within the mixer whichinfluences the mixing/melting efficiency. Since the temperature withinthe mixer is directly proportional to the pressure, i.e. temperatureincreases as more work is performed on the resin, varying the flow ratealso serves to control the temperature. Restraint to the flow ofmaterial discharged from continuous mixers is generally accomplishedthrough the use of an adjustable exit opening and/or by using a movingsurface discharge device which contacts the material with one or moresurfaces moving at controlled speeds in the direction of flow. Gearpumps and extruder screws are most widely used to provide such movingsurfaces.

Mixer melters are typically low energy systems and are not capable ofdeveloping the pressures necessary to achieve acceptable extrusionrates. They are therefore usually coupled with a melt pump, such as agear pump or extruder screw, which imparts the necessary high energy tothe resin melt for extrusion and pelletization. The melt pump alsoserves to control the pressure and throughput in the mixer as describedabove.

While the mixer and the melt pump are independently driven, each havingtheir own motor drive, operation of the mixer and melt pump must becarefully matched to avoid problems in the in-line processing system.For example, if too large an amount of resin melt is fed from the mixerto the melt pump, the pump capacity will be exceeded which can createexcessive back pressure in the mixer causing the rupture disk to blow.If an insufficient amount of resin is fed to the melt pump, i.e. thegear pump is operated in a starved condition, excessive speeds (rpm) canbe generated in the gear pump causing the gear pins to shear. In eitherinstance, the line will have to be shutdown until the necessary repairscan be made. Such disruptions are time-consuming and costly and, if theline is down for an extended period, necessitate shutting down thepolymerizer. To minimize these problems, the mixer and melt pump aresuitably sized and coupled so that feed from the mixer/melter matchesthe requirements of the melt pump but does not exceed that necessary toachieve effective mixing and melting of the resin being processed.

Several methods have been employed by the prior art processes in aneffort to overcome the problems associated with the continuous in-lineprocessing of plastic materials using a mixer interconnected to a gearpump. For example, U.S. Pat. No. 4,310,251 to Scharer et al discloses acontinuous internal mixer which discharges through a fixed-size openingdirectly and positively to either a screw type extruder or a gear pump.The speed of the gear pump or extruder automatically responds to thetemperature of the material discharged from the mixer which is afunction of the mixer internal pressure.

U.S. Pat. No. 4,452,750 to Handwerk et al discloses an in-linemixer-gear pump arrangement for processing synthetic thermoplasticmaterials which employs the pressure developed between the fixed-sizeexit port of the mixer and gear pump to effectively control the speed ofthe gear pump. This in turn affects, in proportional relationship, theamount of energy transmitted to the melter/mixer and the temperature ofthe melt.

U.S. Pat. No. 4,707,139 to Valenzky et al relates to a control systemfor a continuous mixer having a moving surface discharge device. Theinternal pressure in the mixer and the discharge of material from themixer is controlled by the gear pump and the speed (rpm) of the gearpump is controlled in relation to the torque produced by the mixermotor. A change in the torque of the mixer motor results in a change inthe rpm of the gear pump, thereby maintaining the necessary relationshipthroughout the in-line processing operation.

While the above methods generally effectively eliminate the majorproblems associated with such in-line processing operations, they arenot sufficiently responsive to prevent all sudden pressure surges whichresult from unevenness in the mixing action. While these latter pressuresurges are relatively minor compared to the larger catastrophic pressuredifferentials previously mentioned and do not result in equipmentfailure and shutdown of the processing line, they do neverthelessadversely affect the quality of the resulting extruded pellets.Primarily, the pellets are not of uniform size and may be irregularlyshaped. In some instances, the pellets may even agglomerate. While theoversized pellets and agglomerates will be retained upon screening, theymust either be reprocessed or scraped. Smaller-sized pellets will passthrough the screens; and, if they are present in sufficient quantity,necessitate downgrading of the resin.

It is therefore an object of this invention to provide a system andmethod for controlling a continuous mixer having an adjustable exitopening and connected to a moving surface discharge device toeffectively control the speed of the discharge device and therebymaintain the necessary relationships throughout the in-line processingoperation.

It is a further object of this invention to provide a system and methodfor controlling a continuous mixer having an adjustable opening andconnected to a melt pump which is more responsive to changes andimbalances which develop within the in-line processing equipment.

Yet another objective of the present invention is to provide a systemand method for controlling a continuous mixer having an adjustableopening controlled by hydraulic pressure and connected to a gear pump tominimize pressure surges which adversely affect the quality of theextruded pellets and to obtain pellets which are as uniformly sized aspossible.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a process for effectively controlling acontinuous mixer/melter having an adjustable opening and which isconnected to a melt pump. The continuous mixer has interspaced entranceand exit openings and a means for moving the thermoplastic materialtherethrough. Resin melt is discharged from the mixer in a controlledmanner through an adjustable orifice which is opened or closed to adjustthe pressure and temperature within the mixer to an independentlyoperated moving surface discharge device, which can be a gear pump orextruder screw, and which pumps the melt through an extruder die forpelletization. The rate of discharge of melt from the mixer is furthercontrolled by adjusting the speed of the melt pump in proportionalrelation to the energy requirements of the mixer motor, the energyapplied to the adjustable orifice of the mixer and the pressure producedat the interface between the mixer and the melt pump, referred to hereinas the suction pressure. A change in any or all of these variablesresults in a change in the operation of the melt pump, thus compensatingfor changes and imbalances which occur in the system and resulting inmore uniform feed to the extruder die. By eliminating or minimizingtroublesome pressure surges, more uniformly sized resin pellets areproduced.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematized diagram representation of the in-line processingequipment showing arrangement of the mixer/melter and the melt pump andthe relationship of the input and output signals utilized by the systemand method of the present invention.

DETAILED DESCRIPTION

For the purpose of illustration, a specific control configuration is setforth in FIG. 1. Other embodiments are however, possible and the controlconfiguration can be modified accordingly. While reference is made tospecific types of equipment for the processing operation, the presentcontrol system and method can be adapted for use with processingequipment other than that specifically illustrated which accomplishesthe desired function. In general, any control method and system whichdetects changes forward of the suction pressure and anticipates changesin the suction pressure before they occur thereby permitting earlyresponse of the melt pump to compensate for such changes is within thescope of this invention.

In general, input and output signal lines are electrical or pneumaticand can be transduced using any of a variety of transducing means knownin the art. Since transducing means are known and are not a novel partof the present invention, such transducing means are not illustrated andare not described in detail herein. As employed herein, the term"signal" is used in the generic sense and encompasses electrical,mechanical or pneumatic signals or combinations thereof whether indigital or analog form, scaled or unscaled.

To monitor and control the melt pump, a microprocessor is connected tothe melt pump motor in accordance with known procedures. Microprocessorcontrollers useful for the invention may be any of the commerciallyavailable multiple input controllers known to the art. Preferably thecontroller utilizes a proportional-integral or proportional-integralderivative mode of operation. Any controller, however, capable ofaccepting at least three input signals and producing a scaled outputsignal to control a motor can be used and is within the scope of theinvention. Scaling of output signals by controllers is well-known andthe controller may be scaled in any fashion known to effectively controlthe motor. Whereas in the preferred embodiment, the desired torque iscompared with the actual torque by the controller and a signal generatedrepresentative of the change required to equalize the two values, theoutput signal could be based on rpm, flow rate or the like.

The three input signals required for the control of the presentinvention include: the energy required to drive the rotor of themixer/melter; the energy applied to the exit opening of the mixer/melterto maintain the necessary back pressure and temperature within themixer/melter; and the pressure of the thermoplastic resin melt at theinterface between the adjustable exit opening of the mixer/melter andthe inlet of the melt pump. A feedback signal proportional to the actualenergy being used by the melt pump motor is also required. The outputsignal from the processor is utilized to control the speed of the meltpump. In the case of a gear pump, it would control the rpm of the pumpgears.

In the preferred embodiment of this invention, in addition to monitoringthe suction pressure between the mixer/melter and the melt pump, thehydraulic oil pressure of the hydraulic pump utilized to adjust the exitopening of the mixer/melter and the power consumption of themixer/melter motor are monitored. This makes it possible to detectchanges occurring within the mixer/melter at a much earlier stage andbefore these changes affect the suction pressure. Adjustments necessaryto the downstream processing equipment and primarily the melt pump tocompensate for these changes can be anticipated. Therefore, rather thanhaving to recover after demands are made, the downstream processingequipment will already be in a mode to handle these demands when and asthey occur. Specifically referring now to FIG. 1, there is illustratedan in-line processing arrangement in accordance with the presentinvention comprising a conventional continuous mixer/melter 1 directlyand positively connected to a melt pump 10 which feedsextruder/pelletizer 15. Continuous mixer/melter 1 can be anycommercially available mixer/melter. Such continuous mixer/melters aredescribed, for example, in U.S. Pat. Nos. 3,154,808 and 3,237,241. Ingeneral, these mixer/melters include a barrel forming at least onesubstantially cylindrical material working chamber with interspacedentrance and exit openings 8 and 9, respectively. Most generally, suchmixers contain two parallel, laterally interconnected, cylindricalworking chambers having a common exit opening 9. The syntheticthermoplastic resin is introduced through entrance opening 8 where itengages a bladed rotor which extends axially within the cylindricalworking chamber and pushes the material in the direction of exit opening9. When the mixer/melter has multiple working chambers and rotors, therotors are generally geared to rotate in opposite directions. The rotorsmay be configured to create multiple sections or zones within theworking chamber. For example, by varying the blade configurationsuccessive conveying, melting, mixing and pumping zones may be providedwith either single-screw or twin-screw mixer/melters. Such techniquesare well-known and widely used throughout the industry particularly whenthe temperatures of the different zones are also varied.

The thermoplastic material to be processed is fed to the mixer/melter 1through entrance opening 8 via feeder means 7 of the mixer. Feeder 7 canbe a gravity feed or any of a variety of automatic feeders known to theart which are capable of receiving and responding to a control signalwhich can take different forms depending on the particular mode ofoperation. Since the feeder and feeder control do not play a part in thepresent invention and since such feeding mechanisms are known, they arenot described in detail herein.

Mixer/melter 1 has a variable speed motor 2 which drives the bladedrotor within the mixing chamber. The motor 2 is directly coupled to therotor through drive means 3, which can include a gear reducer means orthe like. The motor 2 may be any type, e.g. electric or hydraulic,capable of generating adequate torque and capable of being controlled tomaintain the rotor(s) at constant rpm. The motor 2 is preferably anelectric motor connected to an appropriate power supply L1, L2.

Exit opening 9 is regulated by control means 6 which in addition todetermining the rate of flow of the exiting resin also influences theback pressure and temperature within the mixer/melter 1. The use ofadjustable exit openings on continuous mixers is known and described in,for example, U.S. Pat. Nos. 3,154,808 and 3,237,241. Restriction of theflow of the exiting resin can be accomplished using known designs suchas use of a swinging door or sliding gate type of arrangements.Similarly, the exit openings may consist of multiple ports wherein oneor more of the ports will be opened or closed as required to restrictthe resin flow and control the mixing, as schematically shown at 17. Inyet another arrangement, the continuous mixer/melter can have a movablebarrel which slides in relation to the rotor thereby permitting more orless flow of the resin melt. Continuous mixer/melters of this type areknown and commercially available. All of the above are typicallycontrolled using electric or hydraulic means in response to the pressureor temperature within the mixer/melter. Means for controlling materialflow from mixer/melters in response to signals representative of theflow or conditions within the mixer/melter are known. Similarly, meansfor controlling a mixer/melter motor in response to signalsrepresentative of the flow or pressure or temperature within themixer/melter are known. In view of this and in view of the fact thatneither of these aspects are a novel part of the present invention, suchmeans are not more fully described herein.

Control means 6 is most generally a hydraulic control, i.e. one or morehydraulic cylinders supplied with hydraulic fluid and connected to apressure source, connected to exit opening or discharge orifice 9 toapply opening and closing motion thereto. Since under actual workingconditions the temperature of the thermoplastic material varies quicklyin relation to the amount of work being applied and since thesevariations can be quickly and readily monitored, temperature ispreferably used to control the size of the exit opening 9 and the amountof energy supplied to the motor 2. While the signal for controlling theexit opening 9 and the mixer/melter motor 2 can be generated usingseparate controller means, it can also be accomplished with a singlecontroller. In a preferred mode of operation, the multiple inputmicroprocessor 20 utilized to generate the signal for control of thegear pump motor 11 is also employed to generate the signals whichcontrol mixer/melter motor 2 and hydraulic control means 6 which in turnvaries the size of exit opening 9.

A melt pump having one or more moving surfaces which engage thethermoplastic material is directly and positively connected tomixer/melter 1 after exit opening 9. A direct and positive connection ismade to minimize the distance between the outlet of the mixer/melter andgear pump inlet and to insure that the communication is a hydraulically(resin melt) filled and pressurized communication. Melt pumps with oneor more moving surfaces which contact the resin melt in the direction ofthe flow of the material at a variable and controllable rate are knownfor processing thermoplastic materials and are described, for example,in U.S. Pat. No. 4,707,139. The melt pump 10 is preferably a gear pumpwith inlet directly and positively connected to exit opening 9 andoutlet connected to extruder/pelletizer 15.

Melt pump 10 has a variable speed motor 11 which drives thecounter-rotating intermeshing pump gears. The motor 11 is directlycoupled to the pump gears through drive means 12 which can include agear reducer means or the like. The motor 11 may be any type, e.g.electric or hydraulic, capable of generating adequate torque and capableof being controlled to adjust the gear rpm. The motor 11 is preferablyan electric motor connected to an appropriate power supply L3, L4.

The molten resin exiting the outlet of gear pump 10 is passed through anextruder die plate where the material is pelletized. An extruder die andpelletizing means are shown collectively at 15. Pelletizing means arewell known to the art and generally consist of an extruder plate whichdischarges the extrudate underwater past a chopping means, generallyconsisting of rotating knives or blades. The pellets are cooled andtransported by the water to separator screens to collect the pellets.The pellets are then conveyed to a centrifugal dryer where they aredried and screened to eliminate oversized or agglomerated particles andcollected for storage and transportation.

For the sake of description hereafter, motors 2 and 11 are assumed to beelectric motors; melt pump 10 is assumed to be a gear pump; and controlmeans 6 is assumed to be a hydraulic control operatively connected toexit opening 9 by conventional means known to the art.

In accordance with the present invention there is provided a means formore effectively controlling gear pump 10 to eliminate or minimizechanges in the pressure differential across the pump thereby producingmore uniformly sized resin pellets in extruder/pelletizer 15. This isaccomplished by providing a microprocessor control 20 capable ofreceiving and processing a first input signal indicative of the energyrequired to rotate the rotor(s) of the mixer/melter 1, a second inputsignal indicative of the energy required to vary the size of exitopening 9 and a third input signal indicative of the pressure developedat the interface between exit opening 9 and the inlet of gear pump 10,and generating an output signal to control gear pump motor 11. Afeedback signal from gear pump motor 11 indicative of the actual energyused by motor 11 is also provided to said microprocessor means 20.

The suction pressure, i.e. the pressure developed at the interfacebetween exit opening 9 and the inlet of gear pump 10 will be monitoredusing a pressure detector 41. The signal from the pressure-responsivemeans 41, after transducing from the pneumatic to electric mode, vialine 42, is provided as one of the variable inputs to microprocessor 20.The second or the grove-mentioned signals is similarly generated from apressure sensor 43 and is indicative of the energy required to vary thesize of exit opening 39; and this signal from sensor 43, aftertransducing, is provided as another variable input, via line 44, to themicroprocessor 20. Since the torque developed by motor 2 is proportionedto the amperage drawn, the first of above-mentioned signalsrepresentative of the amperage supplied to motor 2 and is the finalvariable input provided to microprocessor 20. This signal is generatedby sensing means 46, and transmitted to processor 20 via line 47. Itwill of course be understood that alternative forms of these signals canbe used as inputs to microprocessor controller 20 to generate thecontrol signal.

In response to the above-defined input signals, microprocessor 20provides, via line 48, a control signal to motor 11 used to drive gearpump 10 which is representative of the amperage required to maintain therpm of the gear pump 10. A signal which is representative of the actualamperage drawn by motor 11, and thus is indicative of the torquedeveloped by motor 11, in the operation of gear pump 10 is sensed bymeans 50 and fed back to microprocessor 20, via line 51 and comparedwith the target or required value and adjustments made accordingly.

As a result of the process measurements which are used to controloperation of gear pump 10, it has been found that the pump speed (rpm)can be more effectively controlled to, in part, control the energy inputto the thermoplastic material in mixer/melter 1 and further tocompensate for changes and imbalances which occur in the processingequipment forward of the gear pump 10 (primarily in mixer/melter 1 dueto unevenness of mixing action). By detecting changes which occur in theprocessing equipment before the suction pressure and by inputting thesesignals, controller 20 can make adjustments to gear pump 10 tocompensate therefore before they are reflected by a change in thesuction pressure. This permits earlier adjustment of the gear pump 10 sothat it, in effect, anticipates the demands which will be made on it.This has a leveling effect on the overall operation of the process as itcompensates for the generally slow response characteristics of motor 11and gear pump 10.

The thermoplastic resin melt at the outlet of the mixer/melter is,independently, held constant within ±10° C. This temperature will ofcourse vary depending on the particular synthetic thermoplastic materialbeing processed. In general, the temperature is dependent on the amountof energy applied to the thermoplastic mass and the residence time ofmass in the working chamber. The temperature is therefore controlled bythe amount of energy applied to the rotor by motor 2 and the amount ofthe resin exiting mixer/melter 1 through adjustable exit opening 9.Motor 2 and exit opening 9 can be controlled by an independentcontroller or by controller 20 if it has sufficient processingcapabilities and can accept the additional inputs necessary to performthe function and generate additional output signals. The input to thecontroller is generated from a temperature sensor on the mixer/melter 1located near but before exit opening 9. Output signals from thecontroller to motor 2 and hydraulic control means 6 which adjusts exitopening 9 will depend on the difference between the actual temperatureof the thermoplastic resin at the outlet of mixer/melter 1 and thedesired or set point temperature.

I claim:
 1. In the continuous in-line operation of a mixer/melter havingan adjustable size exit opening for feeding a thermoplastic resin meltdirectly and positively to a melt pump which pumps the melt through anextruder/pelletizer for pelletization, to reduce pressure surges andobtain more uniformly sized pellets, the improvement whichcomprises:generating a first input signal representing the amount ofenergy used by the mixer/melter to feed the thermoplastic resintherethrough; generating a second input signal representing the amountof energy used to adjust the size of the adjustable exit opening;generating a third input signal representing the pressure of the resinmelt at the interface between the mixer/melter and the melt pump;transmitting the first, second, and third input signals to a processor;the processor processing the first, second, and third input signals todetermine a desired energy level for the melt pump; and the processorgenerating an output signal and transmitting the output signal to themelt pump to adjust the energy used by the melt pump to said desiredlevel.
 2. In the continuous in-line operation of claim 1, wherein themelt pump includes a pump motor to operate the melt pump, and thetransmitting step includes the step of transmitting the output signal tothe pump motor to adjust the energy used by the pump motor to saiddesired level.
 3. In the continuous in-line operation of claim 2,further including the step of:generating a feedback signal representingan actual torque produced by the pump motor; and transmitting thefeedback signal to the processor; wherein the processing step includesthe step of processing the first, second, and third input signals todetermine a desired torque for the pump motor; wherein the step ofgenerating and transmitting the output signal includes the steps ofi)the processor comparing said actual torque to the desired torque, ii)producing an output signal representing a difference between said actualand desired torques, and iii) transmitting the output signal to the pumpmotor to adjust the torque produced thereby.
 4. In the continuousin-line operation of claim 3, wherein the pump motor is am electricmotor, and wherein:the step of generating the feedback signal includesthe step of generating a feedback signal representing the actualamperage drawn by the pump motor; the step of processing the first,second, and third input signals to determine a desired torque includesthe step of processing the first, second, and third input signals todetermine a desired amperage for the pump motor; the comparing stepincludes the step of comparing the actual amperage to the desiredamperage; the producing step includes the step of producing a controlsignal representing a difference between the actual and desiredamperages; and the step of transmitting the output signal to the pumpmotor includes the step of transmitting the control signal to the pumpmotor.
 5. In the continuous in-line operation of claim 4, furtherincluding the step of, the pump motor adjusting the amperage drawnthereby in response to receiving the control signal.